The present invention generally relates to image forming systems, and more particularly, relates to a highly addressable image forming system employing a printhead.
There are a number of different image forming systems in use today for generating images on a print medium. For example, one of those systems employs focused acoustic energy to eject droplets of marking material, such as ink, from a printhead onto a recording medium. This type of system utilizes printing technology known as acoustic ink printing (AIP) systems.
Printheads utilized in AIP systems most often include a plurality of droplet ejectors, each of which emits a converging acoustic beam into a pool of fluid, e.g., ink. The angular convergence of this beam is selected such that the beam focuses at or near the free surface of the ink, such as at the border between the ink and air. Printing is executed by modulating the radiation pressure that the beam of each ejector exerts against the free surface of ink to selectively eject droplets of ink from the free surface.
In addressable image forming systems that utilize a printhead, such as the AIP printhead discussed above, systematic placement errors can occur in ink droplets. While some errors are random errors, many are repeatable. These systematic placement errors may be caused by manufacturing defects in the printhead or printhead alignment errors, which may result in, drop directionality errors or drop velocity errors. For example, straight vertical lines may look wavy, or in a color image, intercolor bleed is a consequence of ink drop placement errors. Ink droplet placement errors are especially noticeable in an image forming system that employs a bi-directional printhead. This is because a bi-directional printhead ejects ink droplets in opposing directions with each pass across the imaging medium; hence, the ink droplet placement errors are compounded due to opposing printhead directions. As a result of ink droplet placement errors across a printhead, the imaging quality and resolution of a high addressability system, such as an acoustic ink system does not necessarily match the imaging capability of the system.
While some drop position errors are random errors, many of the drop position errors can be predicted and partially corrected in a highly addressable system. One particularly important source of such errors are variations in drop velocity across a printhead. Variations in velocity cause drops from one nozzle to land on the paper sooner than drops from another nozzle. As a consequence, objects, such as lines in the image are more ragged and/or angled differently than intended. These velocity variations can be caused by manufacturing variations in ejector nozzle shape or size. Variability in the ejector shape or size can also result in directionality errors that can cause ink droplet position errors on the imaging media.
An additional cause of ink drop position errors is printhead alignment errors, such as printhead tilt. The amount a printhead tilts into or out of the imaging medium causes differences in the amount of time ink drops take to reach the medium from one end of the printhead to another.
In some instances, ink drop placement variations vary from one page to another due to factors outside of the printhead such as, the thickness of the media. For example, thicker media reduces the amount of time for drops to reach the page and thus the compensation for velocity dependent errors will change. Other factors that cause ink drop placement variations may be transient such as, thermal effects or other variations in the image forming system or within the printhead itself. As long as ink drop placement variations caused by these transient effects are predictable, they can be corrected.
The present invention addresses the above-described ink droplet placement problems across a printhead. In particular, the present invention provides a method for correcting systematic drop position errors in the highly addressable direction. For example, intercolor bleed and wavy lines caused by ink droplet placement errors can be greatly reduced.
In one embodiment of the present invention, a method is performed in an image forming system that discharges ink droplets from the printhead onto an imaging medium to create an image. Once the image is created, differences between a parameter of a first ink droplet and a parameter of a second ink droplet are measured. The parametric measurement of selected ink droplets, such as ink droplet distance from a target point, parallelism between a first ink droplet and a second ink droplet, or a dimensional analysis of the ink droplet on the image medium, is used to derive an ink droplet compensation value for each ink droplet. Once the ink droplet parameters have been measured and the ink droplet velocity compensation values derived, a data file, such as a look-up table that holds the ink droplet compensation values, is created and stored on a storage element. For example, the storage element may be a local hard drive, a semiconductor storage device, such as a RAM device, or as a file on a remote database. A processor utilizes the look-up table to regulate, e.g., to advance or retard, ink droplet discharge from the addressable printhead in order to correct for ink droplet placement errors. In addition, the ink droplet compensation values in the look up table may be adjusted by the user to accommodate for changes in the printing conditions, such as thickness variations in the different imaging media utilized by the image forming system.
The above described approach benefits image forming systems having a highly addressable system. For example, a printhead with a nozzle density of 600 nozzles per inch can fire up to five drops per nozzle per pixel in one printhead scan direction, to produce up to three thousand ink drops per inch. Printhead resolutions equal to or greater than 1200 positions per inch are necessary to make adequate correction possible. It is preferred that the 1200 positions per inch resolution occurs in a single processes direction to insure that corrections are associated with individual ejectors.
Another example would be a 600 dpi printhead that is used to print a 1200xc3x971200 dpi image in two or more passes. On each pass the appropriate correction factor is applied to each ejector to correct for position errors associated with each printhead process direction to within 1200 dpi. The appropriate correction factor is applied to the printhead in the process direction regardless of the number of passes or the size of the printhead advance in the non-process direction. Yet another example would be a 1200 dpi head used to print 1200xc3x971200 dpi in one or more passes. Corrections are made to correct for drop position errors in the same manner.
The ability to control ink droplet discharge occurrences in such an addressable system reduces drop placement errors from an expected forty microns or greater to plus or minus four microns in the high addressability direction. In addition, the reduction in intercolor bleed that can be realized is also advantageous. For example, a system utilizing no ink droplet position compensation generates overlaps between colors varying by more than one pixel. The same printhead that compensates for ink droplet position errors generates variations in overlap between colors only fractions of a pixel, a significant improvement. Further, image forming systems utilizing a bi-directional printhead are especially benefited from this invention, because the compounded ink droplet placement errors that occur in opposing directions, that is left to right and right to left printhead directions, are also reduced. Moreover, stationary printheads, both half page width and full page width, are able to benefit from this method to correct for droplet placement errors that are caused by ink droplet placement variations in the system.
In accordance with another aspect of the present invention, an image forming system includes a printhead facility and a processor for controlling the operation of the printhead. The printhead facility is used to provide the processor with ink droplet position compensation values. The processor utilizes the compensation values to regulate or control the discharge of ink droplets from the printhead. As a result, the regulated or controlled discharge of ink droplets from the printhead corrects for the ink droplet placement errors by advancing or delaying the droplets.
In yet another aspect of the present invention, a method for forming an image is practiced in an image forming system having a highly addressable printhead. The method includes discharging ink droplets from the printhead onto an imaging medium to create an image. Differences between a parameter of a first ink droplet and a parameter of a second ink droplet are then measured.
The differences in the measured parameters are then used to control, regulate, vary or adjust the discharge of ink droplets from the printhead. Further, the velocity or drop direction of the first ink droplet discharged from the printhead relative to the velocity or drop direction of the second ink droplet discharged from the printhead is measured, and any differences or variations in the relative velocities or directions between the first ink droplet and the second ink droplet are controlled or compensated in the highly addressable direction. Moreover, based on the measured differences, the errors caused by the tilt of the printhead are compensated for, and one or more ejectors of the printhead are used to normalize the direction and speed of the ink droplets relative to one another. Lastly, the differences in the measured parameters caused by an air gap distance between the printhead and the imaging medium, or time effects, or thermal effects of the printhead may also be used to control, regulate, vary or adjust the discharge of ink droplets from the printhead.
In accordance with an other aspect of the present invention, a method for forming an image with a printhead in an image forming system is performed. First the image forming system discharges a first set of ink droplets and a second set of ink droplets from the printhead. Then differences are determined in spacing between the first set of ink droplets and the second set of ink droplets on the imaging medium. The determined differences are then used to control, regulate, vary or adjust the discharge of the ink droplets from the printhead based on the differences in distance.
In accordance with a further aspect of the present invention, an image forming system includes a printhead, a processor for controlling the printhead, and a printhead facility coupled to the processor for controlling the printhead based on differences between a parameter of a first ink droplet and a parameter of a second ink droplet discharged from the printhead. Based on the differences between the parameter of the first ink droplet and the parameter of the second ink droplet, the processor varies the discharge from the printhead during an imaging operation. Moreover, the parameter differences may include drop position data corresponding to at least one of the first ink droplets and at least one of the second ink droplets. Further, where the printhead includes one or more ink ejectors, the processor in conjunction with the print head facility adjusts one or more of the ink ejectors as a function of the measured parameter differences.